This invention relates to methods and apparatus for the assessment of the composition of a liquid, in particular to the detection of impurities present in a liquid, with particular, but by no means exclusive, reference to the detection of impurities present in samples of water.
There is an unfulfilled need for accurate, sensitive and reliable monitoring of water quality. Preferably, a single technique would exhibit wide ranging sensitivity, permitting the detection of a range of pollution types. A variety of substances can cause pollution, including oil, sewage, organic wastes as well as a multitude of industrial by-products. When the water being monitored is a flowing source, such as a river, industrial effluent, sewage flow, tap water or groundwater source, the technique employed is preferably an on-line one. The present invention is primarilyxe2x80x94although not exclusivelyxe2x80x94directed towards on-line monitoring of such flowing sources.
Techniques which have been employed for these purposes include: the inhibition of bacteria, the use of fish, changes in acoustic impedance, optical monitoring, and measurement of biological oxygen demand (BOD), chemical oxygen demand (COD), pH, turbidity and conductivity. The latter technique, that of conductivity measurement, is of the greatest pertinence to the present invention. Such measurements have been used for a number of years to determine water quality, and involve the measurement of the conductance of a water sample when a direct current (dc) electrical signal is applied thereto. The conductance of the water sample is dependent upon the chemical species contained in the sample, and therefore the measured value of the conductance can be taken as an indication of the degree of pollution. However, the technique only provides a single experimental quantityxe2x80x94the dc conductancexe2x80x94and thus the amount of information provided by the technique, concerning the identity and concentration of any pollutants, is limited. Furthermore, correlating variations in conductivity with the presence of pollutants is not an easy matter, since conductivity variations can be caused by normally occurring, natural variations in the composition of the water sample. Such compositional variations can occur over a time scale of minutes, as well as having seasonal causes.
The present invention alleviates the aforementioned problems. For the avoidance of doubt, it is noted that the invention is applicable inter alia to the detection of a wide range of impurities in liquids, including samples of waterxe2x80x94flowing and stillxe2x80x94emanating from numerous sources, and also including non-conductive liquids, such as oils.
According to a first aspect of the invention there is provided a method for assessing the composition of a liquid comprising the steps of:
applying an electrical or electromagnetic signal to the liquid; and
measuring an impedance quantity at, or near to, the resonant frequency of the liquid or an electrical circuit comprising the liquid so that the resonant frequency or variations in the resonant frequency, can be detected.
The impedance quantity may be measured as a function of the frequency of an applied electrical signal.
A time varying electrical signal may be applied to the liquid.
The time varying electrode signal may be an ac signal and the frequency of the ac signal may be varied.
Alternatively, the measurement of an impedance quantity may comprise a time to frequency domain transformation of the time varying electrical signal. Said signal maybe periodic, and may comprise a pseudo random binary sequence (PRBS) code, a Golay code, a Walsh Function, a Huffman sequence or any other suitable coded sequence. Other suitable signals, codes, or methodologies such as white Gaussian noise or wavelet analysis, may be employed.
A high power electrical signal may be applied to prevent or reduce bio-fouling thereof (i.e. the coating of the electrodes with biomass).
The electrical signal may be applied via one or more electrodes or windings which are not in direct electrical contact with the liquid. The one or more electrodes or windings which are not in direct electrical contact with the liquid may be encased within a non-conductive material, and disposed in the liquid. Alternatively, they may be positioned around the liquid. An advantage with the indirect electrical contact with the liquid is a reduction in, or elimination of, bio-fouling processes.
The impedance quantity measured may be a quantity directly related to the resonant frequency such as the dissipation factor. Data obtained in this manner are highly dependent upon the nature of the impurity, and furthermore, provide very sensitive impurity detection. An inductor may be used to adjust the resonant frequency.
A microwave electromagnetic signal may be applied to the liquid.
The method may further comprise the step of analysing the measurement of an impedance quantity with artificial intelligence means, which means may comprise an artificial neural network.
Alternatively, the method may further comprise the step of analysing the measurement of an impedance quantity with reference to a look up table.
Said steps of analysing the measurement of an impedance quantity may account for the effect of temperature on the measurement. Alternatively, the temperature of the liquid may be controlled.
The presence of one or more impurities in the liquid may be detected.
The liquid may be water, and may be a flowing source, such as a river, industrial effluent, sewage flow, tap water or a groundwater source. The method may be performed in order to detect the presence of pollutants in a water sample. On-line measurements may be made.
The analysis of the measurement of an impedance quantity may be such that normally occurring variations in the measured impedance quantity are recognised.
Measurements may be made at a plurality of locations, and data relating to the measurements may be relayed to a central location.
The liquid may be non-conductive, such as an oil. The quality of the oil may be monitored.
The presence of a microorganism in the liquid may be detected.
The liquid may be a beverage or a foodstuff.
According to a second aspect of the invention there is provided apparatus for assessing the composition of a liquid comprising:
electrical signal applying means adapted to apply a time varying electrical signal to the liquid; and
measuring means for measuring an impedance quantity at, or near to, the resonant frequency of an electrical circuit comprising the liquid so that the resonant frequency, or variations in the resonant frequency, can be detected.
The electrical signal applying means may apply an ac signal of variable frequency. The measuring means may comprise an impedance analyser.
Alternatively, the measuring means may perform a time to frequency domain transformation of the time varying electrical signal, which may be periodic.
The electrical signal applying means may be in direct electrical contact with the liquid.
The electrical signal applying means may comprise at least two electrodes in direct electrical contact with the liquid.
Alternatively, the electrical signal applying means may not be in direct electrical contact with the liquid. A portion of the electrical signal applying means may be encased within a non-conductive material, said portion being disposed in the liquid, or the electrical applying means may be positioned around the liquid. The electrical signal applying means (when not in direct electrical contact with the liquid) may comprise one or more electrodes, or at least two windings.
The apparatus may further comprise temperature control means adapted to maintain the liquid at a substantially constant temperature.
The apparatus may further comprise artificial intelligence means for analysing the measurement of an impedance quantity. The artificial intelligence means may be an artificial neural network.